Identification of a Novel Factor V A2 Domain Tyrosine Deletion Mutation in a Patient with Factor V Deficiency and Bleeding.

The gene for coagulation Factor V (FV) is located on chromosome 1q23. FV deficiency shows an autosomal recessive mode of inheritance; heterozygotes are generally not clinically affected. The homozygous clinical phenotype occurs in approximately 1 per million individuals with variable severity of bleeding. Thus, genotype-phenotype correlations are likely to shed light on functionally important residues of FV. Here we describe a case of FV deficiency with a severe bleeding phenotype. The proband is a male infant from Argentina. His parents are unrelated. He was born healthy with no bleeding from the umbilical stump or other symptoms. He presented at eight months with a CNS hemorrhage, then suffered a second massive subdural bleed at nine months of age. Both episodes required surgical drainage and treatment with fresh frozen plasma He continues to receive prophylactic FFP infusions and has some residual neurologic impairment. The proband’s FV activity ranges from 2–14%. Two siblings are unaffected. His father’s FV activity is 50% and his mother’s is 70%. We performed DNA sequencing spanning the entire coding region of the proband’s FV gene and found two heterozygous mutations: a heterozygous single base pair deletion, del 2952T in exon 13, located in the B-domain of the FV protein, causing a frameshift mutation followed by a premature termination codon 3 amino acids downstream; and a novel 3-bp deletion in exon 10. This deletion is in-frame and results in the deletion of Y478. The del 2952T frameshift mutation was present in the father, while the del Y478 mutation was present in the mother. Y478 is in the A2 domain of FV and adjacent to another tyrosine, Y477. Evidence suggests that these tyrosine residues are important for co-factor function. Tyrosine residue sulfation has been shown to be required for full activity of the homologous co-factor, FVIII, as well as for hirudin. These sulfated tyrosines and surrounding acidic amino acids have been proposed to be important in interactions with the thrombin anion binding exosite; in the case of hirudin, sulfation of a carboxy-terminal tyrosine increases the affinity for thrombin 10-fold. The homologous tyrosines, Y718 and Y719 appear to be sulfated in FVIII. FV has been shown to be sulfated, but the precise location of the FV sulfation sites has not yet been determined. One of this patient’s FV alleles is nonfunctional due to a frameshift and a premature trancation of translation. With respect to the other allele, we hypothesize that, like FVIII, one or both of FV tyrosines 477 and 478 is sulfated, and that deletion of Y478 may result in disruption of FV co-factor function. In vitro mutagenesis and expression studies to characterize the functional consequences of the del Y478 and/or del Y477 are in progress.